Field of the Invention
This invention relates to a control device for a vehicle, and particularly to a control device for a vehicle equipped with an internal combustion engine including port injection valves that inject fuel into an intake passage.
Description of the Background Art
Japanese Patent Laying-Open No. 2011-073474 discloses a hybrid vehicle equipped with an internal combustion engine including port injection valves and in-cylinder injection valves. The document discloses that in this hybrid vehicle, the engine is operated under load when target power to be output from the engine is not smaller than a prescribed value, and the engine is operated on its own or is stopped operating when the target power is smaller than the prescribed value.
For a low pressure fuel system in which fuel is supplied into the port injection valves, the pressure regulator method has been conventionally used. In order to improve fuel efficiency, however, the use of a variable fuel pressure system with less energy loss has been considered.
In the pressure regulator method, the pressure of the fuel is stabilized by excessively pressurizing the fuel with a fuel pump, and discharging part of the fuel through a pipe using a pressure regulator valve. On the other hand, in the variable fuel pressure system, which has improved accuracy of pressurizing the fuel pump, fuel may be directly supplied to the fuel pipe without using the pressure regulator valve.
In a hybrid vehicle, fuel efficiency is improved by causing the engine to be shut down while the vehicle is stopped, or causing the vehicle to run (EV running) while the engine is shut down. In this way, the engine is used as required. The engine is therefore operated intermittently. Here, the intermittent operation of the engine may cause the fuel pressure in the variable fuel pressure system to rise above an original target value. When the engine is stopped from its operating state, the injection of the fuel from the port injection valves is stopped. At this time, if the timing of stopping the fuel feed pump is late, the fuel pressure is increased above the target value. Furthermore, because the fuel is not discharged from a low-pressure delivery pipe, the fuel stored in the low-pressure delivery pipe may expand due to the absorption of heat from the engine. This may cause the fuel pressure to increase even though the fuel feed pump is stopped.
Under a high fuel pressure, a minimum injection amount of the fuel that can be injected from the port injection valves also increases. When the minimum injection amount is increased, the injection amount during operation with a small amount of air becomes excessive relative to the amount of air. As a result, the air-fuel ratio becomes fuel-rich to cause exhaust deterioration. This operation with a small amount of air tends to occur at the start of the engine during the intermittent operation of the engine.